Evaporated surfaces, dislocations

Evaporated surfaces, dislocations on, 19 331 Evolution, of catalysts, extended X-ray absorption fine structure studies in, 35 101 Ewald constructions, 21 174, 175 EXAFS, see Extended X-ray absorption fine structure... 

Figure C2.11.6. The classic two-particle sintering model illustrating material transport and neck growtli at tire particle contacts resulting in coarsening (left) and densification (right) during sintering. Surface diffusion (a), evaporation-condensation (b), and volume diffusion (c) contribute to coarsening, while volume diffusion (d), grain boundary diffusion (e), solution-precipitation (f), and dislocation motion (g) contribute to densification.

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

Surface features can also be revealed by etching, which permits identification of points of intersection of line dislocations with the surface, and this is valuable in determining the role of these imperfections in chemical processes [45,214] and, in particular, nucleus formation. Smaller topographical details can be rendered visible by the evaporation of a thin (<0.5 nm) film of gold onto the surface [215,216]. Heights and depths of surface features can be determined by interferometry [203—205]. Microcinematography has also been used [217] to record the progress of solid phase reactions. 

Since the evaporation of a solid would occur at the kink sites because the bonding is weaker, atoms would diffuse also to these sites before evaporation. A demonstration of this is to be found on the morphologies of single crystals after a period of heating in vacuum to cause substantial evaporation. The resultant surface shows an increase in the number of ledges and kinks relative to the area of the terraces. It is also to be expected that dislocations emerging at the surface of catalysts, either as edge or screw dislocations, would play a... 

Because TiB2 belongs to the covalently bonded solids, the intrinsic diffusivity is very low and therefore the Peierl s stress is high for the movement of dislocations. The preferential neck growth in mass transfer was affected by evaporation-condensation and surface... 

Figure 4.19 Illustration of a possible mechanism for top-metal penetration upon evaporation of the top metal contact onto the SAM. The SAM exhibits pinhole defects that can occur at grain boundaries where the natural tilt of the molecules (-20° off the surface normal) are dislocated at differing domains and at step edges where there are single atomic step defects in the underlying metal substrate. These pinhole defects could be the source of the metal shorts upon evaporation or continued usage of the devices. The larger the area, the more defects that will be present, thereby exacerbating the top metal punch through problem.

Experimental evidence of neck growth was first demonstrated by Kuczynski, who in 1949 sintered large polycrystalline particles onto flat polycrystalline substrates. Theories, based on a two sphere model (each a single crystal), were developed to determine the rate of neck growth and the rate at which particle centres approach one another. These theories conclude that the rate of neck growth is inversely proportional to particle size raised to a power that depends on the mass transport path. Many mass transport paths were subsequently considered, bulk diffusion, surface diffusion, grain boundary diffusion, viscous flow, evaporation-condensation, liquid solution-reprecipitation, and dislocation motion (see reference 8 for a review). 

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